The present invention relates to an improved method and apparatus for locking shape charges into a charge holder for use in well perforation operations. The improved apparatus eliminates the need to bend tabs on the charge holder to hold the charges in position.
The performance of an underground well, such as a hydrocarbon producing well, can be improved by perforating the formation containing the hydrocarbons. Well perforation operations involve the controlled detonation of shape charges within the well. The shape charges perforate the casing, if any, and the surrounding formation, thereby improving the flow of hydrocarbons from the well. A perforation gun is used to hold the shape charges. The perforation gun is lowered into the well on either tubing or a wireline until it is at the depth of the formation of interest. The gun assembly includes a charge holder that holds the shape charges and a carrier that protects the shape charges from the environment. A detonation cord links each charge located in the charge holder.
A shaped charge is inserted into a mating hole of the charge holder, and a charge retention apparatus holds the charge firmly within the mating hole. The charge retention apparatus normally include retaining rings, charge retention jackets, clips, or bending tabs, all of which are designed to secure the shaped charge in the charge holder. Thus, it is desirable to provide a method and apparatus for securing the shaped charge to the charge holder of a perforating gun without using a separate charge retention apparatus or bendable tabs.
An example of a common charge retention design is shown in FIGS. 1 to 5. A perforation gun 10 is shown having a charge holder 12 and a cover (not shown). The charge holder has at least one opening 16 for receiving the shape charge 18. Each shape charge 18 has a case 20 that is generally conical in shape. An explosive charge is nestled into the case 20. Most cases 20 also have a pair of tabs 22 that retain the detonation cord 24. The detonation cord 24 can ignite the explosive charge through a detonation transfer passage 32 through the case 20. The carrier can have scalloping 26 that corresponds to the location of each charge. The scalloping is an area of reduced thickness used to minimize any resistance to the exploding shape charge.
The gun is assembled by sequentially placing the charges 18 into the charge holder 12. Once inserted, the charge must be secured in place. At least one deformable tab 30 can be used. The tip of a tool such as a screw driver is inserted into slot 28, as shown in FIG. 2, and the tab 30 is deformed until it is in contact with the front of the charge case. There are obvious drawbacks to such a retention scheme. For one, fabrication of the slot 28 is expensive and time consuming. The charge holders 12 are typically made of steel and the slots require an additional fabrication step to cut. Further, the deformation of the tab 30 takes additional assembly time. It also causes a minor deformation to the rest of the charge holder. When many tabs are deformed, the overall charge holder can experience a meaningful length increase. The deformation of the charge holder can also cause a misalignment of the charges and scalloping 26 on the carrier. Equally troublesome is the inability to easily disassemble the loaded charge holder. If the perforation gun is lowered into a well and for an unknown reason fails to fire, the gun is removed from the well and disassembled. This requires a worker to remove the charge holder from the carrier and then use a tool to undeform the tabs 30 to remove the charge. The use of the tool around the live charges introduces a safety concern. Also, once disassembled, the charge holder is unusable and must be scrapped.
A need exists for an easier and less expensive method to load charges into a charge holder. Such a method should use an improved charge holder design that does not require the use of deformable tabs. One attempt at making such a charge holder is shown in U.S. Pat. No. 5,952,603 to Parrott and entitled xe2x80x9cInsert and Twist Method and Apparatus for Securing a Shaped Charge to a Loading Tube or a Perforating Gun.xe2x80x9d Rather than the use of tabs, Parrott ""603 discloses the use of specially designed lugs on the charge case. In FIGS. 6 and 7 illustrate an embodiment of the Parrott ""603 design. A shaped charge case 52 is inserted into a mating hole 54 of a loading tube 50 prior to inserting the loading tube in a perforating gun carrier. The pair of retaining lugs 52a are inserted into slots 54a and 54b of the mating hole 54 and, simultaneously, the pair of shoulder lugs 52c are inserted into the first and second pair of grooves 54c and 54d, the lugs 52c being initially inserted into the large diameter groove L associated with the first and second pair of grooves 54c, 54d. At this point, a wrench is required. The wrench is secured to the pair of support lugs 52b and twisted clockwise. The clockwise torque provided by the wrench on the support lugs 52b moves the shaped charge case 52 in a clockwise circumferential direction.
In FIG. 7, during the movement of the shaped charge case 52 in the clockwise circumferential direction, an end portion of the retaining lugs 52a move underneath a surface of the loading tube 50. Simultaneously, the pair of shoulder lugs 52c move out of the large diameter groove L of the pair of grooves 54c and 54d and into the small diameter groove S of the pair of grooves 54c and 54d. In this position, the surface of the loading tube 50 prevents the retaining lugs 52a of the shaped charge case 52 from moving in an outward radial direction; and the small diameter groove S prevents the shoulder lugs 52c from moving in an inward radial direction. In addition, the small diameter groove S of the first and second pair of grooves 54c and 54d prevents the shaped charge case 52 from moving either clockwise or counterclockwise in a circumferential direction.
The Parrot ""603 design is exceedingly difficult and expensive to fabricate. For example, most charge cases are made of either steel or zinc. To place the lugs on a steel case requires a welding step or a very expensive lathing process. Zinc can be cast, and therefore to be cost effective, one is essentially limited to the use of a zinc case. Therefore, a need exists for a simpler and less expensive apparatus for holding shape charges in a carrier. Such an apparatus should not require expensive milling steps to construct and should quick to assemble and disassemble. In the event of disassembly, the carrier should be reusable.
The present invention addresses many of the drawbacks found in prior art retention schemes. In one embodiment, the invention uses a pair of opposed tabs on the carrier to engage a groove in the case of the shape charge. The case is generally conical in shape. However, the front opening of the case has both a first and second circumferential ridge separated by a groove. The diameter of the groove generally matches the distance between tabs. The second ridge can have a flat milled thereon to allow case to be inserted until the tabs contact the forward or first ridge. This locates the tabs between the first and second ridges and over the groove. The case is then rotated so that the tabs are captured between the two ridges. In another embodiment, there is no forward ridge, and the tab only contacts the rear ridge.
Once installed, the detonation cord is attached to the case. The cord helps prevent any rotation of the case, thereby securing it into the charge holder. Thus, the use of easily milled tabs on the charge holder in conjunction with an easily milled groove and flats on the case provide an improved apparatus and method for securing the shape charge into the charge holder of a perforation gun. When the charges are all installed, and the detonation cord is properly linked between each charge, then the charge holder can be placed in a carrier.
If a need arises for disassembling the perforation gun, the charge holder can be removed from the carrier. The detonation cord is then uncoupled from the charges to be removed. The charge case can then be simply turned until the tabs on the charge holder are aligned with the flats on the first ridge. The charge case is then pulled from the charge holder, leaving the charge holder in condition to be used again. No tools are required for this operation.